To protect global environment, ceramic honeycomb filters for removing carbon-based particulate from exhaust gases discharged from diesel engines are used. Because a ceramic honeycomb filter comprises a ceramic honeycomb structure having cell walls constituting large numbers of flow paths, whose inlets and outlets are alternately plugged, the cell walls are clogged with a large amount of particulate captured, resulting in high pressure loss. It is necessary to burn the captured particulate to regenerate the ceramic honeycomb filter. The ceramic honeycomb filter is thus required to be able to capture particulate and withstand the repeated burning of particulate, so that the ceramic honeycomb filter have high heat resistance and heat shock resistance. Accordingly, it has conventionally been made of cordierite (5SiO2-2Al2O3-2MgO). However, the ceramic honeycomb filter is likely heated to temperatures exceeding 1450° C., the melting point of cordierite, by heat generated by the burning of particulate. When the temperature of the filter exceeds the melting point of cordierite, the cell walls are partially melted, resulting in a low particulate-capturing ratio. Investigation has thus been made to form a ceramic honeycomb structure with aluminum titanate (Al2TiO5) having a melting point of about 1860° C.
Although aluminum titanate has a high melting point and a small thermal expansion coefficient, it has a large anisotropy in a thermal expansion coefficient, resulting in high likelihood of generating micro-cracks between crystal grains, and thus decreased mechanical strength. In addition, the aluminum titanate crystals have low thermal stability, because they are easily decomposed to TiO2 (rutile) and Al2O3 (corundum) at temperatures of 750-1200° C.
JP 62-40061 B discloses a ceramic honeycomb having high heat resistance, high porosity, high compression strength and low thermal expansion coefficient, which is made of 85% or more by mass of aluminum titanate and 4-10% by mass of SiO2, and which has a thermal expansion ratio of 0.15% or less when heated to 1000° C., compression strength of 350 kg/cm2 or more (converted to the value when an opening ratio is 0), and porosity of 35% or more. JP 62-40061 B describes that aluminum titanate synthesized in the presence of SiO2-containing clay (crystalline clay such as kaolin) is preferably used to produce the ceramic honeycomb. However, a ceramic honeycomb made of aluminum titanate synthesized in the presence of clay does not have sufficient thermal stability at temperatures of 750-1200° C.
JP 7-25662 A describes that the sintering of ceramic powder comprising 0.3-5.6% by mass of SiO2, and 0.7-14.4% by mass of Al2O3, the balance being aluminum titanate, provides composite ceramics of aluminum titanate and mullite having improved mechanical strength and high-temperature thermal stability, without losing a high melting point and a low thermal expansion coefficient inherent in aluminum titanate. It is described that SiO2 added to aluminum titanate may be crystalline such as quartz, tridymite, cristobalite, etc. or amorphous, and that Al2O3 added to aluminum titanate may be crystalline such as α-Al2O3, γ-Al2O3, etc. or amorphous. However, mullite-containing sintered aluminum titanate produced by adding SiO2 powder and Al2O3 powder to aluminum titanate powder is disadvantageous in insufficient thermal stability, and a large thermal expansion coefficient because of a large thermal expansion coefficient of mullite.
The aluminum titanate synthesized in the presence of crystalline SiO2 (JP 62-40061 B), and the mullite-containing aluminum titanate produced by adding amorphous or crystalline SiO2 powder and Al2O3 powder to aluminum titanate powder (JP 7-25662 A) have thermal stability improved to some extent compared with aluminum titanate without SiO2, but it is still not enough to solve the problem that aluminum titanate is thermally decomposed to TiO2 (rutile) and Al2O3 (corundum) at temperatures of 750-1200° C. Accordingly, ceramic honeycomb structures made of aluminum titanate as a main component have not been put into practical use yet.